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See Mridha & Kümmerli, Commun. Biol.
An open question in microbiology is whether seemingly coordinated group-level responses, such as biofilm formation, actually mirror what individual cells do. To tackle this question, the Kümmerli group used single-cell microscopy to simultaneously quantify the investment of individual P. aeruginosa cells into two public goods. Using gene expression as a proxy for investment, these bacterial cells initially show no coordination in siderophore investment, but rather high heterogeneity and bi-modality. However, with increasing cell density, gene expression becomes more homogenised across cells, with positive associations in siderophore gene expression across cells and with cell-to-cell variation correlating with cellular metabolic states. This suggests that siderophore-mediated signalling aligns behavior of individual bacteria over time and spurs a coordinated three-phase siderophore investment cycle, covering the time spans from low to high population density, steered by the various interconnected regulatory mechanisms governing siderophore synthesis.